Porous fin as compliant medium to form dislocation-free heteroepitaxial films

ABSTRACT

A method for forming a heteroepitaxial layer includes forming an epitaxial grown layer on a monocrystalline substrate and patterning the epitaxial grown layer to form fins. The fins are converted to porous fins. A surface of the porous fins is treated to make the surface suitable for epitaxial growth. Lattice mismatch is compensated for between an epitaxially grown monocrystalline layer grown on the surface and the monocrystalline substrate by relaxing the epitaxially grown monocrystalline layer using the porous fins to form a relaxed heteroepitaxial interface with the monocrystalline substrate.

BACKGROUND

Technical Field

The present invention relates to semiconductor processing, and moreparticularly to methods and devices that employ porous materials as amedium for addressing lattice mismatch between crystalline materials.

Description of the Related Art

Heteroepitaxy is the process of epitaxially growing one crystal onanother. Since the crystal structures are usually different, issues canarise involving lattice mismatch between the different crystalstructures. These differences are responsible for the formation ofdislocation defects. Si wafers are preferred as a seed layer for growingother epitaxial films. Si is usually chosen due to cost-effectivenessconsiderations and due to its being a well-established material platformfor thin film processing. Lattice mismatch between Si and other filmsinduces dislocation-formation, which affects further fabrication anddevice performance.

SUMMARY

A method for forming a heteroepitaxial layer includes forming anepitaxial grown layer on a monocrystalline substrate and patterning theepitaxial grown layer to form fins. The fins are converted to porousfins. A surface of the porous fins is treated to make the surfacesuitable for epitaxial growth. Lattice mismatch is compensated forbetween an epitaxially grown monocrystalline layer grown on the surfaceand the monocrystalline substrate by relaxing the epitaxially grownmonocrystalline layer using the porous fins to form a relaxedheteroepitaxial interface with the monocrystalline substrate.

Another method for forming a heteroepitaxial layer includes forming anepitaxial grown p+ doped silicon layer on a monocrystalline siliconsubstrate; forming fins by patterning the epitaxial grown silicon layerwherein the fins are sized to provide compliance to permit an amount ofrelaxation; anodizing the fins to form porous silicon fins; performing ahydrogenation process on a surface of the porous silicon fins to makethe surface suitable for epitaxial growth; growing an epitaxialmonocrystalline layer on the surface; and relaxing the epitaxialmonocrystalline layer by employing elasticity of the porous siliconfins.

A semiconductor device includes a porous fin formed on a monocrystallinesubstrate and a hydrogenated surface formed on the porous fin. Anepitaxial monocrystalline layer is formed on the hydrogenated surface.The epitaxial monocrystalline layer includes a material other than amaterial of the substrate and forms a relaxed heteroepitaxial interfacewith the monocrystalline substrate.

These and other features and advantages will become apparent from thefollowing detailed description of illustrative embodiments thereof,which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The disclosure will provide details in the following description ofpreferred embodiments with reference to the following figures wherein:

FIG. 1 is a cross-sectional view of a substrate having a p+ doped layerepitaxially grown therein in accordance with the present principles;

FIG. 2 is a cross-sectional view of the substrate of FIG. 1 wherein thep+ doped layer is patterned to form fins therein in accordance with thepresent principles;

FIG. 3 is a cross-sectional view of the substrate of FIG. 2 showing thep+ doped fins converted to porous silicon by a conversion process suchas anodization in accordance with the present principles;

FIG. 4 is a cross-sectional view of the substrate of FIG. 3 showing thefins converted to porous silicon being treated to form a surfacesuitable for epitaxial growth in accordance with the present principles;

FIG. 5 is a cross-sectional view of the substrate of FIG. 4 showing amonocrystalline epitaxial layer formed on the porous silicon fins inaccordance with the present principles;

FIG. 6 is a magnified cross-sectional view of the substrate of FIG. 5showing the monocrystalline epitaxial layer spontaneously relaxing usingthe compliance of the porous silicon fin in accordance with the presentprinciples;

FIG. 7 is a cross-sectional view of the substrate of FIG. 5 showing themonocrystalline epitaxial layer being laterally expanded by epitaxialgrowth in accordance with the present principles;

FIG. 8 is a cross-sectional view of the substrate of FIG. 7 showing themonocrystalline epitaxial layer being vertically expanded by epitaxialgrowth in accordance with the present principles; and

FIG. 9 is a block/flow diagram shows methods for forming aheteroepitaxial layer in accordance with the present principles.

DETAILED DESCRIPTION

In accordance with the present principles, methods for heteroepitaxialgrowth and devices made by these methods are provided. Theheteroepitaxial growth processes begin with a substrate, preferably ofSi, and form fins on or in the substrate. The fins are converted toporous material (for example, porous Si (PS)) using, e.g., an anodizingprocess). Then, the fins are processed to form a surface layer suitablefor continued epitaxial growth. In one embodiment, the porous Si finsare treated using a hydrogenation process to form a continuous Si shellon the porous Si fins. An epitaxial process is employed to grow adifferent crystalline material on the surface of the fins.

The different crystalline material is relaxed by the compliance of theporous material of the fin. For example, an elastic modulus of porous Siis a fraction (e.g., between about ⅓ to about ⅕) of typicalsemiconductor materials. The compliance of the fins relaxes thedifferent crystalline material. In this way, dislocation defects fromlattice mismatch are reduced or eliminated.

Porous Si fins can be semi-freestanding bodies to accommodate latticemismatch with other epilayers formed thereon. By controlling a thicknessratio between porous Si fins and the epilayers formed thereon, strain inthe epilayer can be fully-relaxed. Porous Si can be stretched or shrunkto follow the lattice of epilayers. In this way, spontaneous relaxationof the porous Si can prevent dislocation formation.

It is to be understood that the present invention will be described interms of a given illustrative architecture; however, otherarchitectures, structures, substrate materials and process features andsteps may be varied within the scope of the present invention.

It will also be understood that when an element such as a layer, regionor substrate is referred to as being “on” or “over” another element, itcan be directly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “connected” or “coupled” to another element, it canbe directly connected or coupled to the other element or interveningelements may be present. In contrast, when an element is referred to asbeing “directly connected” or “directly coupled” to another element,there are no intervening elements present.

The present embodiments may be employed in a design for an integratedcircuit chip, which may be created in a graphical computer programminglanguage, and stored in a computer storage medium (such as a disk, tape,physical hard drive, or virtual hard drive such as in a storage accessnetwork). If the designer does not fabricate chips or thephotolithographic masks used to fabricate chips, the designer maytransmit the resulting design by physical means (e.g., by providing acopy of the storage medium storing the design) or electronically (e.g.,through the Internet) to such entities, directly or indirectly. Thestored design is then converted into the appropriate format (e.g.,GDSII) for the fabrication of photolithographic masks, which typicallyinclude multiple copies of the chip design in question that are to beformed on a wafer. The photolithographic masks are utilized to defineareas of the wafer (and/or the layers thereon) to be etched or otherwiseprocessed.

Methods as described herein may be used in the fabrication of integratedcircuit chips. The resulting integrated circuit chips can be distributedby the fabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged chips), as a bare die, or in a packaged form. In thelatter case the chip is mounted in a single chip package (such as aplastic carrier, with leads that are affixed to a motherboard or otherhigher level carrier) or in a multichip package (such as a ceramiccarrier that has either or both surface interconnections or buriedinterconnections). In any case the chip is then integrated with otherchips, discrete circuit elements, and/or other signal processing devicesas part of either (a) an intermediate product, such as a motherboard, or(b) an end product. The end product can be any product that includesintegrated circuit chips, ranging from toys and other low-endapplications to advanced computer products having a display, a keyboardor other input device, and a central processor.

It should also be understood that material compounds will be describedin terms of listed elements, e.g., GaN. These compounds includedifferent proportions of the elements within the compound, e.g., GaNincludes Ga_(x)N_(1-x) where x is less than or equal to 1, etc. Inaddition, other elements may be included in the compound, such as, e.g.,AlGaN, and still function in accordance with the present principles. Thecompounds with additional elements will be referred to herein as alloys.

Reference in the specification to “one embodiment” or “an embodiment” ofthe present principles, as well as other variations thereof, means thata particular feature, structure, characteristic, and so forth describedin connection with the embodiment is included in at least one embodimentof the present principles. Thus, the appearances of the phrase “in oneembodiment” or “in an embodiment”, as well any other variations,appearing in various places throughout the specification are notnecessarily all referring to the same embodiment.

It is to be appreciated that the use of any of the following “/”,“and/or”, and “at least one of”, for example, in the cases of “A/B”, “Aand/or B” and “at least one of A and B”, is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of both options (A andB). As a further example, in the cases of “A, B, and/or C” and “at leastone of A, B, and C”, such phrasing is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of the third listedoption (C) only, or the selection of the first and the second listedoptions (A and B) only, or the selection of the first and third listedoptions (A and C) only, or the selection of the second and third listedoptions (B and C) only, or the selection of all three options (A and Band C). This may be extended, as readily apparent by one of ordinaryskill in this and related arts, for as many items listed.

Referring now to the drawings in which like numerals represent the sameor similar elements and initially to FIG. 1, a partially fabricatedsemiconductor device 10 is illustratively shown in accordance with oneembodiment. Device 10 includes a substrate 12, which preferably includesmonocrystalline material. The substrate 12 may include Si, althoughother substrate materials may be employed, e.g., SiGe, SiC, Ge, III-Vmaterials, II-VI materials, etc. An epitaxially grown layer (epilayer)14 is formed on the substrate 12. In one embodiment, the epilayer 14includes a p+ doped silicon material (e.g., p+ refers to p-type dopantswith a doping concentration greater than 5×10¹⁸/cm³). The epilayer 14preferably includes Si for the formation of porous Si; however, theepilayer 14 may include other materials as well. For example, porousforms of Ge or III-V materials may also be employed. For simplicity,further description will focus on an illustrative porous Si embodiment;however, other materials may be employed.

Referring to FIG. 2, the epilayer 14 is patterned to form fins 16. Thefins 16 are preferably patterned using a lithographic mask and etchingprocess. The fins 16 are etched using a reactive ion etching process,although other types of etching processes may be employed. Thedimensions of the fins 16 are determined based upon a thickness ratiothat will be employed in ensuring an amount of relaxation of anepitaxial grown layer on the fin 16. For example, a fin's dimensions maybe determined based on the amount of compliance needed. In oneembodiment, a thickness of a fin is needed to be less than its height.For example, if the height is 30 nm or more, the thickness should be 30nm or less.

Referring to FIG. 3, the fins 16 are converted to porous silicon (PS)fins 18. The conversion may be performed by employing an anodizationprocess, strain etching, drying or other techniques. In a particularlyuseful embodiment, anodization is employed. An anodization cell isemployed that may include a cathode (e.g., platinum) and the dopedsilicon fins 16 as an anode. In one embodiment, the fins 16 are immersedin an HF electrolyte. Corrosion of the anode is produced by runningelectric current, preferably direct current (DC), through the cell toensure a more homogenous porosity. The porosity of the fins 18 may be40% or more, although other porosities may also be employed.

Referring to FIG. 4, the porous silicon fins 18 are treated to provide asurface suitable for epitaxial growth. In one embodiment, the poroussilicon of fins 18 is exposed to a hydrogenation process. Thehydrogenation process exposes the porous silicon fins 18 to hydrogen toprovide a continuous silicon surface 20 on exterior portions of the fins18. The hydrogenation process includes exposing the porous silicon tohydrogen ions at a temperature of between about 500 degrees C. and about900 degrees C. to remove the pores on the surface.

The hydrogenation process includes annealing in an environmentcontaining hydrogen. Si atoms at surfaces reflow to seal the pores atthe surface to form a continuous Si layer (like the surface of singlecrystalline silicon). In illustrative embodiments, temperature, hydrogenpressure and annealing time are processing parameters that can becontrolled to achieve a suitable surface 20 on the fins 18. The pressuremay range from about 50 milli-Torr to about 100 Torr. The annealing timemay range from about 5 seconds to about 200 seconds.

Referring to FIG. 5, the continuous surface 20 is suitable for epitaxialgrowth. An epitaxial layer 22 is grown on the continuous surface 20. Thethickness ratio between the fins 18 and the epitaxial layer 22 is basedupon the material being grown. The fin material 18 needs to havesufficient dimensions to fully (or partially) relax the material of theepitaxial layer 22. For example, if the material being grown (epitaxiallayer 22) needs a 1% strain to be relaxed, the dimensions of the finneeds to be able to provide the 1% compliance to achieve fullrelaxation. It may be useful in some embodiments to maintain a portionof the strain depending upon the particular application.

Referring to FIG. 6, the epitaxial layer 22 grown may include, e.g.,GaN. The strain in the GaN layer 22 is relieved by stretching the fins18 in the direction of arrows 24. GaN stretches because the poroussilicon core has a very low Young's modulus and thus high compliance.The fins 18 need to be able to handle at least the relaxationdeflection. In other embodiments, the direction of relaxation may beperpendicular to arrows 24. In still other embodiments, the relaxationmay be in multiple directions. The thickness ratio between the fins 18(fin thickness) and the epitaxial layer (22) thickness may be betweenabout 2:1 to about 1:5.

Referring to FIG. 7, in one embodiment, the epitaxial layer 22 may beformed and employed as depicted in FIG. 6. In other embodiments, theepitaxial growth process may be continued to laterally grow theepitaxial material to form a layer 26 of epitaxially grown material thatfills in spaces between the fins 18. The layer 26 may be employed informing diodes, photovoltaic devices, lasers, transistors, etc.

Referring to FIG. 8, in another embodiment, the epitaxial growth processmay be continued to vertically grow the epitaxial material to form athicker layer 28 of epitaxially grown material. The layer 28 may beemployed in forming diodes, photovoltaic devices, lasers, transistors,etc. The exemplary structures of FIG. 6, FIG. 7 and FIG. 8 are growndislocation-free using the compliant fins 18 formed from porous silicon.

It should be understood that porous silicon may be replaced with othercompliant materials that can bond with a substrate and can form asurface suitable for epitaxial growth. Examples may include porous Ge,porous III-V material, etc.

The epitaxial layer 22 may include any epitaxially grown material.Examples may include II-VI materials, III-V materials, IV-IV materials,etc. Specific examples of the epitaxial layer 22 may include GaN, AN,ZnO, SiC, InN, BeSe, ZnS, MgS, BeTe, MgSe, AlP, GaP, AlAs, ZnSe, Si, Ge,InP, GaAs, CdSe, ZnTe, InGaAs, etc. These and other materials may begrown dislocation free using spontaneous relation processes inaccordance with the present principles.

Referring to FIG. 9, methods for forming a heteroepitaxial layer areillustratively depicted. In some alternative implementations, thefunctions noted in the blocks may occur out of the order noted in thefigures. For example, two blocks shown in succession may, in fact, beexecuted substantially concurrently, or the blocks may sometimes beexecuted in the reverse order, depending upon the functionalityinvolved. It will also be noted that each block of the block diagramsand/or flowchart illustration, and combinations of blocks in the blockdiagrams and/or flowchart illustration, can be implemented by specialpurpose hardware-based systems that perform the specified functions oracts or carry out combinations of special purpose hardware and computerinstructions.

In block 102, an epitaxial grown layer is formed on a monocrystallinesubstrate. The epitaxial grown layer preferably includes a same materialas the substrate. The epitaxial grown layer may be doped (e.g., withp-type dopants such as boron with a doping concentration greater than5×10¹⁸/cm³) to enhance its anodic properties (over the substrate) if ananodizing process is layer employed. Since Si is a commonly processedmaterial, in one embodiment, the substrate and the epitaxial grown layerinclude monocrystalline silicon, although other materials may beemployed. In block 104, the epitaxial grown layer is patterned to formfins. The patterning process may include lithography. In otherembodiments, the fins may be selectively grown on the substrate, e.g.,epitaxially grown through a patterned layer. Other processes for formingthe fins may also be employed. In block 105, the fins are sized toprovide compliance to permit an amount of relaxation of the epitaxiallygrown monocrystalline layer.

In block 106, the fins are converted to porous fins. The conversionprocess may include anodization, strain etching, drying or othertechniques, with anodization being preferred (using the fins as ananode). In block 108, a surface of the porous fins is treated to makethe surface suitable for epitaxial growth. Treating the surface of theporous fins may include performing a hydrogenation process to form acontinuous surface. Other suitable processes may also be employed.

In block 110, an epitaxial monocrystalline layer is grown on the surfacewherein lattice mismatch is compensated for between the epitaxiallygrown monocrystalline layer grown on the surface and the substrate, byrelaxing the epitaxially grown monocrystalline layer using the porousfins. The porous fins provide compliance to form a relaxedheteroepitaxial interface with the monocrystalline substrate. Theelasticity of the porous silicon fins is employed as a relaxationmedium. Compensation for lattice mismatch or relaxation of strain mayinclude stretching or shrinking the porous fins to relax the epitaxiallygrown monocrystalline layer.

In block 112, the epitaxially grown monocrystalline layer may beextended by lateral growth to form a continuous layer. In block 114, theepitaxially grown monocrystalline layer may be vertically grown to forma thicker layer. If combined with block 112, a thicker horizontallycontinuous dislocation-free epitaxially grown monocrystalline layer canbe provided. In block 116, processing may continue to from a device ordevices. For example, the epitaxially grown monocrystalline layer may beemployed to form any type of device that employs or would be desirableto employ a heteroepitaxial interface. These devices may include, e.g.,transistors, diodes, lasers, etc.

Having described preferred embodiments porous fin as compliant medium toform dislocation-free heteroepitaxial films (which are intended to beillustrative and not limiting), it is noted that modifications andvariations can be made by persons skilled in the art in light of theabove teachings. It is therefore to be understood that changes may bemade in the particular embodiments disclosed which are within the scopeof the invention as outlined by the appended claims. Having thusdescribed aspects of the invention, with the details and particularityrequired by the patent laws, what is claimed and desired protected byLetters Patent is set forth in the appended claims.

The invention claimed is:
 1. A method for forming a heteroepitaxiallayer, comprising: forming an epitaxial grown layer on a monocrystallinesubstrate; patterning the epitaxial grown silicon layer to form fins;converting the fins to porous fins; treating a surface of the porousfins to make the surface suitable for epitaxial growth; and compensatingfor lattice mismatch of an epitaxially grown monocrystalline layer grownon the surface by relaxing the epitaxially grown monocrystalline layerusing the porous fins to form a relaxed heteroepitaxial interface withthe monocrystalline substrate.
 2. The method as recited in claim 1,wherein forming the epitaxial grown layer includes forming an epitaxialgrown p+ doped layer.
 3. The method as recited in claim 1, whereinpatterning the epitaxial grown layer to form fins includes sizing thefins to provide compliance to permit an amount of relaxation of theepitaxially grown monocrystalline layer.
 4. The method as recited inclaim 1, wherein converting the fins to the porous fins includinganodizing the fins using the fins as an anode.
 5. The method as recitedin claim 1, wherein treating the surface of the porous fins includesperforming a hydrogenation process to form a continuous surface.
 6. Themethod as recited in claim 1, wherein compensating for lattice mismatchincludes stretching or shrinking the porous fins to relax theepitaxially grown monocrystalline layer.
 7. The method as recited inclaim 1, further comprising laterally growing the epitaxially grownmonocrystalline layer to form a continuous layer.
 8. The method asrecited in claim 7, further comprising vertically growing theepitaxially grown monocrystalline layer to form a thicker layer.
 9. Amethod for forming a heteroepitaxial layer, comprising: forming anepitaxial grown p+ doped silicon layer on a monocrystalline siliconsubstrate; forming fins by patterning the epitaxial grown silicon layerwherein the fins are sized to provide compliance to permit an amount ofrelaxation; anodizing the fins to form porous silicon fins; performing ahydrogenation process on a surface of the porous silicon fins to makethe surface suitable for epitaxial growth; growing an epitaxialmonocrystalline layer on the surface; and relaxing the epitaxialmonocrystalline layer by employing elasticity of the porous siliconfins.
 10. The method as recited in claim 9, wherein relaxing includescompensating for lattice mismatch between the monocrystalline siliconsubstrate and the epitaxial monocrystalline layer.
 11. The method asrecited in claim 9, wherein relaxing includes stretching or shrinkingthe porous silicon fins.
 12. The method as recited in claim 9, furthercomprising laterally growing the epitaxial monocrystalline layer to forma continuous layer.
 13. The method as recited in claim 12, furthercomprising vertically growing the epitaxial monocrystalline layer toform a thicker layer.